This invention relates generally to magnet structures for a displacement sensor and, more particularly, to such structures which generate in an air gap a magnetic field which varies linearly over a measurement axis.
Magnetic displacement sensors are known which consist of one or more magnets producing an induction field B and of an element for measurement of the induction (Hall probe or magnetoresistance, for example) which are set in relative motion. The resolution of such a sensor is proportional to the sensitivity of the element for measuring the induction and to the gradient of this induction in the displacement range. This gradient depends on the nature of the magnets, on the form of the magnetic circuit and on its position with respect to the measurement element.
The most common devices of this type consist of a magnet attached to the device whose position must be measured. In this case, the weight of the magnet must be relatively low so as not to disturb the movement of the device.
When the probe is placed opposite the magnet, the variation of the magnetic field as a function of the displacement is large and not linear. The signal of the probe must then be processed after it is compared to reference signals. This type of device, for example, permits measurements of displacement on the order of 100 mm with a precision of 0.1 mm for a Samarium-cobalt magnet volume of 400 mm.
When the probe is placed in a zone where the field varies linearly, and if J designates the polarization axis of the magnet, the probe may be placed parallel to the axis J and the magnet moved along this axis, or the probe may be placed so it is perpendicular to axis J. The signal of the probe is then proportional to the displacement, and the induction gradient depends on the form and the nature of the magnet, as well as on the distance from the magnet to the probe. The signal is on the order of 0.1 T/mm to 1 T/mm over a distance of 0.1 to several millimeters.
The displacement measurement devices which apply these arrangements of probes and magnets are sensitive to a ferromagnetic environment. One proposed structure is a sensor of microdisplacements with magnetic circuits which provide in an air gap a large variation of induction over a measurement range from 100 to 500 .mu.m, approximately.
Another proposed structure consists of a permanent magnet and a yoke in the form of a U, both associated, to form a closed magnetic circuit. The position of the sensor along the yoke is determined as a function of the intensity of the leakage field detected by the sensor, and the shape of the yoke can be adjusted to obtain a linear variation of the field.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.